Method and apparatus for edge grinding small objects



H. F. FRUTH Oct. 16, 1945.

METHOD AND APPARATUS FOR EDGE GRINDING SMALL OBJECTS Filed June 25', 1943 2 Sheets-Sheet 1 IN V EN TOR.

H. F. FRUTH Oct. 16, 1945.

METHOD AND APPARATUS FOR EDGE GRINDING SMALL OBJECTS 2 Sheets-Sheet 2 INVENTOR. Hal F Trad/a BY Filed June 25, 1943 Patented Oct.- 16, 1945 METHOD AND APPARATUS FOR EDGE GRINDING SMALL OBJECTS Hal F. Fruth, Chicago, 111., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois 4 Application June 25, 1943, Serial No. 492,203

9 Claims. (Cl. 51-164) The present invention relates to methods and apparatus for grinding the edges of small objects and, more particularly, to improved methods and apparatus for finish grinding theedges of piezoelectric crystals in order to remove irregularities therefrom and to produce polished edge surfaces thereon.

In the manufacture of certain articles or parts which have relatively sharp edges formed at the intersections. between the surfaces of each part or article, it is frequently desirable, if not essenmain at the edges of the crystal have a tendency to break away, causing crystal dust which remains on the surfaces of the crystals. The dust particles will sometimes change the frequency of vibration ofthe crystal or cause the vibration of the crystal to be entirely arrested, which of course is undesirable from an operating standpoint.

tial, that the edges of the article be ground for the purpose of removing irregularities'therefrom- Thus in the manufacture of piezoelectric crystals adapted for use in communication circuits and, more particularly, for use in crystal microphones, radio transmitting and receiving systems and the like, the crystal blanks are cut from the crystal stock, are ground to the dimensions required to provide the desired frequency characteristics, and are then edge groundto remove irregularities from the edges thereof. The usual crystal blank is cut in the form of a. fiat piece and is then reduced to rectangular dimensions slightly larger than the desired dimensions by grinding the sides and ends thereof on a. suitable grinding wheel. According to conventional practice, all finish grinding operations are performed by hand, and are eilected by bringing the desired surface or edge to bear against an abrasive surface and manually moving the bearing surface of the crystal across the abrasive surface. grinding operations are performed in two steps,

These finish i, e. face grinding and edge grinding, either of which may be performed ahead of the other; Both operations, as practiced by conventional manual methods, aretediously slow and require skilled labor in the performance thereof. Also, in the finish grinding of crystals for use in ultra high frequency circuits having a thickness of seven mils or less, the waste due to breakage during the finish grinding of the crystals by hand, is unduly high. Moreover, the finished crystals obtained by. using such grinding methods are not entirely satisfactory in operation. The operating difficulties which have been experienced are attributable in large measure to the fact that it is apparently impossible with conventional edge grinding methods toobtain' crystals which are free ofedge irregularities. Under certain temperature conditions, those edge irregularities or discontinuities which are present in a finished crystal tend to produce spurious vibrating frequencies which are diiiicultto stop when once started. Moreover, the minute points which re- It is an object of the present invention, there'- fore, to provide improved methods and apparatus for finish grinding the edges of small objects or articles to remove edge discontinuities therefrom.

It is another object of the invention to provide methods and apparatus for grinding piezoelectric crystalsin an improved manner such that the disadvantages of the manual grinding methods now commonly inuse are obviated, and an improved crystal structure having more stableoperatingcharacteristics is obtained.

According to another object of the invention, a, controlled method is provided, which may be utilized on a variable time basis with a expenditures of manual labor, to edge grind small ob ects so that they conform with precision accuracy to a predetermined standard. Y 1

It is still another object of the invention to minimize the amount of labor and abrasive material required to edge grind the objects in production quantities.

It is a still further object of the invention to provide an improved method and apparatus for edge grinding piezoelectric crystals'in batches at high speeds and with aminimum of manual labor. v v

The invention, both as. to its organization and method of operation, together with further objects and advantages thereof,- 'w ill best be understood by reference to the following specification taken in connection with the accompanying drawings, in which: 1 f

Fig. 1 is a side view of improved apparatus which is characterized by thel-featuresof the present invention and may be' utihzed to practice the improved method blanksorthelike;

. Fig. 2 is an end sectionalview of't'he apparatus showninFig.1; Y

Fig. 3 is a face view of afcrystal which has been expenditure of. .edge gringiing' crystal Fig. 5 is an end sectional view of the grinding tube shown in Fig. 4;

Fig. 6 is a side view illustrating a modified arrangement of the edge grinding apparatus;

Fig. '7 is an end sectional view the apparatus shown in Fig. 6; and

Fig. 8 is an end sectional view illustrating the manner of mounting the grinding tubes forming a part of the apparatus shown in Figs. 6 and 7.

Referring now to the drawings and more particularly to Figs. 1 and 2 thereof, the improved edge grinding apparatus there illustrated comprises a base HI having a U-shaped carriage ll tiltably mounted thereon within two upstanding bracket pieces l2 and I3. Side members it and i5 which are flanged at the ends thereof to overlie the end panels Ila and lib of the carriage ll are provided for rotatably supporting the carriage upon the two bracket pieces l2 and M.

More specifically, the flanged portions of the two side members M and I5 are riveted to the carriage panels Ila and l lb adjacent the tops thereof and carry pivot pins II and I! at aligned central points thereof, which are suitably journaled in bearing members respectively carried by the two bracket pieces l2 and il. The carriage II is arranged detachably to support a grinding tube assembly 9 which comprises six grinding tubes I! of the same length and internal diameter. These tubes are arranged with their long axes in parallel and offset from the center of rotation of the tubes. They may be formed of glass, metal or any other material having the required hardness to resist abrasion. Rollers l9 and are utilized to rotatably support the tubes l8 upon the carriage H. To this end, six equiangularly spaced-apart and aligned openings are cut through the two rollers l9 and 20 for receiving the six tubes. For the purpose of rotatably supporting the rollers l9 and 20 upon the carriage H two driving rollers 2 la and 2H; rigidly mountedupon a shaft 22 at opposite ends of the carriage. H, and two idler rollers 23 mounted upon a shaft 24 at opposite ends of the carriage H,

are employed. The two shafts 22 and 24 are suitably journaled in the top corner portions of the carriage end panels lid and Nb.

A driving motor 25, which is mounted upon the floor lie of the carriage H, is, provided for im-. parting rotary movement to the rollers l9 and 20, thereby torotate the grinding tubes 18 about the longitudinally aligned centers of the two rollers. This motor is equipped with a motor pulley 28 which is connected by a driving belt 21 to drive a second pulley 28 rigidly mounted upon the roller drive shaft 22. In order to continuously tilt the carriage ll between two extreme positions, wherein the grinding tubes I! are oppositely inclined with respect to the horizontal, a second motor 29 is provided, which is crank connected to the end panel llb of the carriage. More specifically, the rotor shaft of this motor carries a wheel 30 having an eccentric pin 3| mounted thereon which is journaled within one end of a crank arm 32. The opposite end of the arm 32 is pivotally connected to the carriage end panel lib by axis means comprising the two bearing brackets 33 and 34 and a pivot pin 35. As thus arranged, rotary movement of the wheel 30 serves continuously to reciprocate the carriage H between two extreme positions wherein the grinding tubes IB respectively occupy the positions shown in full lines and in dash lines in Fig. l of the drawings.

ass-mac readily removed for the purpose of inserting orthat rubber grommets 38 may be inserted within the roller openings. The tubes 18 may then be inserted through the aligned grommet openings of the two rollers so that a semirigid assembly is provided which is completely removable from the supporting rollers 2| and 28.. In this regard it will be noted that the upper inner edge portions of the two carriage and panels Ho and Ill; serve as friction bearing surfaces against-which the side surfaces of the rollers I9 and 20 may bear to prevent movement of the grinding tube assembly 9 in a direction longitudinal of the grinding tubes. In other words, with the tube assembly positioned upon the rollers 2| and 23 of the carriage, the engagement of the rollers l8 and 20 with the upper inner edge portions of the end panels Ho and llb serves to prevent the tube assembly from sliding oil of the carriage.

In utilizing the above-described apparatus for the purpose of grinding the edges of piezoelectric crystals or other small objects, the crystals are disposed within the tubes It. For the purpose of retaining the crystals or other objects within the tubes, each tube is provided at each end thereof with a rubber stopper 31 which may be withdrawing crystals from the tube chamber. In order to produce random change in the position of each crystal relative to the tube within whichit is confined,'a bailie or obstruction 3| is provided at one or more points each along each tube. Each home may comprise a small narrow strip of adhesive, tape extending longitudinally of the tube in themanner shown in Figs. 4 and 5 of the drawings. Depending upon the type'of random movement of the crystals which is desired, the

formed by removing the grinding tube assembly 9 from the carriage and holding the same in any angular position which is convenient for loading the individual tubes with'a charge of crystals and abrasive material. The grinding tube assembly is then positioned upon the carriage with the rollers l9 and 20 in engagement with the driving rollers 2| and the idler rollers 23. The motors 25 and 29 are now energized for the purpose of simultaneously rotating the tubes l8 and tilting these tubes back and forth so that the angle of inclination of the tubes with respect to the horizontal is continuously changed. As the tube as- Tgsembly 9 is tilted, each crystal within each tube is slid longitudinally from one end of the enclosing tube to the other. The rotary movement of the tubes causes each crystal to slide over the inner surface of theenclosing tube in a substantially circular path. As a result of the combined the other is in the form of a helix. The manner in which the positions of the crystals with respect to their respective enclosing tubes are changed at random and the edge grinding operations are carried out, is more fully explained below.

In the modified arrangement of the apparatus shown in Figs. 6 and 7 of the drawings, the grinding tube assembly 9 is made up of five grinding tubes I8 which are radially offset from an assembly rod 39 having two sets of tube receiving clips 40 and 4I mounted thereon at spaced-apart positions therealong. These two sets of tube receiving clips are each provided with five pairs of clip fingers which are formed of brass or other spring-like material. The fingers of each pair are so shaped that when distended or forced apart, they will snugly embrace a tube inserted therebetween. Thus, the two sets of clips 40 and H, as rigidly mounted upon the rod 39, are utilized to support the five grinding tubes I8 in equiangularly spaced-apart positions for rotation about the center of the rod 39. The tube assembly is supported between .the rotor shaft 42 of a driving motor 43 and a rotor plate 44. More specifically, the rod 39 carries two cover plates 45 and 46 at opposite. ends thereof between which the ends of the tubes I8, as closed by the stoppers 31, are adapted to be received. The projecting right end of the rod 39 may extend through any one of a plurality of radially aligned openings 41 drilled in a metal bearing plate 48. This plate is rigidly mounted upon the inner surface of the rotor plate 44 to extend radially outward from the center thereof. The openings 41 are each of .a diameter slightly larger than the diameter of the rod 39 and register with openings of greater diameter cut through the rotor plate 44, so that a loose universal connection is provided between the end of the rod 39 and the rotor plate 44. The rotor plate 44 is rotatably mounted at the center thereof upon a stub shaft 49 which is carried by a bracket lili suitably mounted upon a base As shown in Fig. 7 Of the drawings, the bracket 50 also mounts a motor 52 having an actuating roller 53 mounted upon the rotor shaft thereof which engages the periphery of the rotor plate 44 for the purpose of rotating this plate.

At its left end, the assembly rod 39 is supportedby one-half of a universal connection 54, the other half of which is rigidly mounted for rotation with the-shaft 42 of the motor 43. If

ings 41, the diameter of the circle described by the right end of the tube assembly may be changed as desired for the purpose of adjusting the extent to which the assembly is tilted with respect to the horizontal. In loading the tubes I8 of the Fig. 6 arrangement with a suitable abrasive material and with crystals which are to be edge ground,- these tubes may be individually withdrawn from the retaining fingers of the clips 40 and 4| and unstoppered for the purposeof in serti'ng the desired charge of crystals and abra-' siv'e material therewithin. In this regard it is pointed out that each of the tubes I8, as provided in the embodiment of the apparatus shown in Figs. 6 and 7, is, like the tubes shown in Fig. 1, equipped with one or more-baflle strips 38 for the purpose of changing the position of each crystal within its enclosing tube in a manner more fully pointed out below. i

The present improved method of edge grinding, as practiced by utilizing either embodiment of the improved apparatus described above, may conveniently be considered in its application to the finish grinding of the edges of piezoelectric quartz crystals. As previously indicated, such crystals are initially sliced from the mother crystal after which they are rough ground to the approximate [iii desired, a short rubber tube may be employed as I 9 about the axis of the rotor plate 44 so that the' angle of inclination of this assembly with respect to the horizontal is alternately reversed during alternate half revolutions of the rotor plate 44.

In this regard it will be understood that during operation of the apparatus, the two motors 43 and 52 are concurrently energized so that the tubes III are rotated about the assembly rod 39 and the right end of the tube assembly 9 is rotated about the center of the rotor plate 44. By suitably inserting the right end of the assembly rod 38 in a selected one of the bearing plate opendesired rectangular dimensions on a grinding wheel. After the rough grinding operations are completed, two finish grinding operations are re quired, namely, the grinding of the crystal faces to the desired thickness for the purpose of obtaining the desired frequency characteristics, and the edge grinding of the crystals for the purpose of removing discontinuities therefrom. These two finish grinding operations may be performed in any desired order but both are necessary in order to obtain a satisfactory crystal structure. In the usual crystal structure, the crystallength is somewhat greater than the crystal width. These. dimensions are taken into' account in determining the inner diameters of'the tubes l8 which are utilized in the above-described apparatus. More specifically, the purpose of the edge grinding operation is that of producing a rounded chamfer or bevel at the junction point between each pair of intersecting surfaces of each crystal. Accordingly, the diameter of each tube I8 is properly chosen to produce the desired average chamfer radius at the edges defining the intersections between the crystal surfaces. Thus, to provide a relatively broad chamfer, it is necessary that the tube diameters rather closely approximate the longest dimension of each crystal. 0n the other hand, to produce a sharp chamfer, the tube diameters should be relatively large as compared with the longest dimension of each crystal. Preferably, the dimensions of the crystals as related. to the tube diameter are about as indicated in Fig. 5 of the drawings. As there shown, the diameter of the illustrated tube I8 is so chosen that when a crystal is supported at its edges with it short axis extending longitudinally of the tube, a second crystal may be supported thereabove with the long axis thereof extending transversely of the tube-and with the adjacent surfaces of the two As indicated above, to condition either embodiment of the disclosed apparatus for a grinding operation, the tubes l8 are each loaded with a batch of crystals and a charge of abrasive material. The abrasive material may comprise a small amount of diamond powder or boron carbide, the particular grinding speed being determined to a large extent by the size of the grinding particles employed. Thus. if rapid grinding is desired, diamond powder of No. 100 to No. 150 screen mesh may be used. On the other hand, if less rapid grinding and a higher polish of the chamfer surfaces is to be obtained, No. 320 to No. 400 screen mesh grinding material may be utilized. The amount of grinding material used' in each tube will of course depend upon the dimensions of the tube. For example, when tubes having a length of about 60 inches and an internal diameter of 1.0625 inches are employed, satisfactory grinding is obtained if approximately two carats of No. 100 to No. 150 screen mesh diamond powder is used in each tube. On the other hand, if the finer grinding powder of from No. 320 to No. 400 screen mesh is employed, approximately two to three carats of the grinding material should be used in each tube.

After each tube is loaded with a batch of.

crystals and a charge of abrasive material and the tubes are suitably associated with the moving means of either of the two disclosed embodiments of the apparatus, operation of the driving motors of either embodiment may be initiated forthe purpose of initiating the edge grinding operation. The abrading action which occurs is substantially similar in each of the two embodiments of the apparatus and, accordingly, will be considered with specific reference to the apparatus shown in Figs. 1 and 2 of the drawings. Due to the continuous and reverse tilting movement of the tubes l8, each crystal within each tube is slid longitudinally of the tube over the inner abrasively coated surface thereof. Concurrently therewith, and due to the rotation of the tube assembly 9 by the motor 25, the tubes are rotated about the axial centers of the rollers l9 and 20, so that the inner surface of each tube is slid beneath the edge surfaces of each crystal which it houses. After rotation and reciprocation of the tube assembly is started, the crystals tend to align themselves along the lengths of their respective enclosing tubes with only the straight edge sections thereof bearing against the inner surfaces of their enclosing tubes. The abrasive material is of course soon spread over the inner surface of each tube to produce an abrasive surface a ainst which the edge portions of the crystals bear. Also, the contact pressure at the points of engagement of the edges of a particular crystal with the abraslvely coated inner surface of its enclosing tube is primarily determined by the weight of the crystal. This is due to the fact that the longitudinal movement of the crystals through the tubes tends to prevent the crystals from becoming stacked one above the other. Dueto the abrasive action of the abrasive material, the high points of the crystal are cut away as the crystals are slid over the surfaces of their enclosing tubes. In this connection it is pointed out that the movement of the crystal edges longitudinally of the tubes prevents transverse scratches from being produced in the chamfered surfaces which are soon formed at the edges of each crystal. Similarly, the sliding of the crystal edges circumferentially of the abrasive surfaces against which they bear prevents the chamfered surfaces from being scratched longitudinally incident to the sliding of the crystals back and forth within their enclosing tubes.

Preferably, the speed at which the tilting motor 29 is operated is so related to the lengths of the tubes 18 and the angle of inclination of the tube assembly with respect to the horizontal that a crystal disposed at the elevated end of a tube will just slide to the other end of the tube before the angle of inclination of the tube is reversed. It has been found that if tubes fifty-eight inches long are employed and the maximum angle of inclination with respect to the horizontal is five degrees and forty-two minutes, a crystal will be slid from one end of an enclosing tube to the other if the tilting motor 29 is operated at a speed of one revolution per minute. The grinding .tubes are rotated at a much faster speed, a speed of from eighty to ninety revolutions per minute having been found to be satisfactory in practice. It will be understood, therefore, that the maximum abrading action is produced by rotation of the tubes to slide the abrasively coated inner surfaces thereof beneath the edges of the crystals which are supported upon these surfaces. However, the longitudinal movement of the crystals prevents the transverse scratches from being produced in the chamfered edge surfaces during the rapid rotation of the abrading surfaces.

The longitudinal movement of the crystals along the tubes is also utilized to change the positions of the crystals within the tubes so that different edge sections thereof are brought to bear against the abrasively coated inner surfaces of the tubes. crystal tumbling movement, it may be assumed that the uppermost crystal as shown-in Fig. 5 of the drawings is slid into a position where it is transversely aligned with the baffle strip 38. As the illustrated tube l8 in which this crystal is disposed is rotated, one edge of the strip 38 engages one side of the crystal so that during coning this period the crystal is not only turned over so that alternate faces thereof face downwardly,

but in addition is turned end for end so that all eight of the edges thereof are at one time or another brought to bear against the abrasively coated inner surface of the tube. Incident to the From the above explanation, it will be understood that the edge grinding of the crystals-dis posed within the tubes 18 is produced with a completely random action; that is, the back and forth movement of the crystals with respect to the abrasive surfaces does not follow any predetermined or ascertainable pattern but differs for To consider one aspect of the crystals and the abrading surfaces, the surfaces of the chamfers produced on the crystals are almost entirely free of scratches or other irregularities and are highly polished. The uniformity and highly polished character of the chamfered surfaces which are produced by utilizing the present improved method and apparatus are exemplified by the finished crystal illustrated in magnified form in Fig. 3 of the drawings. From an examination of this crystal, it will be seen that all of the edges and corners thereof are smoothly rounded and are entirely free of scratches and other irregularities. The method is also characterized by the advantage that practically no surface or face grinding of the crystals is produced, with the result that the resonant frequency characteristic of each crystal is substantially unal tered during the edge grinding operation. By virtue of this fact, the edge grinding may be performed either before or after the faces of the crystal are finishground to impart predetermined frequency characteristics thereto. In other words, the absence of face grinding permits great latitude in the order in which the crystals are ground and permits alterations in the order of the finish grinding steps to suit the grinding requirements of a'particular batch of crystals.

Although the invention has been described with particular reference to the edge grinding of piezoelectric crystals of rectangular configuration, it will be understood that the apparatus and method may be employed with equal facility in grinding to predetermined dimensions the edges of relatively fiat objects of practically any pattern and material. For example, the disclosed apparatus and method are well adapted for use in the edge grinding ofoptical lenses of round, oval or other configuration. They may also be used in the edge grinding of small fiat metal parts of any configuration. It is also pointed out that the tubes may be constructed to have different cross-sectional configuration. For example, tubes having diamond-shaped cross-section may be used. In such case, the width of each flat surface should be less than the smallest ilatwise dimension of the particular article upon which the chamfered edges are to be produced. As an alternative to the use of grinding powders or other loose granular abrasive materials, the grinding tubes, regardless of the configuration thereof, may be lined with a suitable abrasive paper or cloth. In such case, the abrasive paper or cloth is provided in strips and is secured to the inner surface of each tube in spiral fashion with the adjacent edges abutting each other in the same manner as if the material were wound spirally around the outer surface of a tube. When sandpaper or abrasive cloth is used, suitable baille strips 38 are provided at appropriate points along the abrasive surface for the purpose of producing the random change in the positions of the crystals relative to the abrasive surface as the crystals are slid longitudinally from one end of each tube to the other. If the sand paper or cloth has sufliclent stiffness, the tubes l8 may be made up of such material, without any backingv surface whatever.

While different embodiments of the invention have been disclosed, it will be understood that various modifications may be made therein, which are within the true spirit and scope of the invention.

I claim:

1. The method of grinding the edges of a substantially fiat object, which comprises freely supporting the object within a closed tube having an abrasive surface at the inner'side thereof with only the edges of the object bearing against said surface, tilting said tube at a predetermined speed so that one section thereof is alternately above and below the other section thereof, thereby to slide said object back and forth over said surface, and rotating said tube at a speed several times greater than said predetermined speed to produce further sliding movement of said object relative to said surface in a direction normal to the direction of said first-named sliding movement, whereby the length' of the path traversed by said object in moving from one end of said tube to the other is greatly in excess of the length of said tube.

2. The method of grinding the edges of 'a substantially flat object, which comprises freely supporting the object within a closed tube having an abrasive surface at the inner side thereof with only the edges of the object bearing against said surface, tilting said tube at a predetermined speed so that one section thereof is alternately above and below the other section thereof, thereby to slide said object back and forth over said surface, rotating said tube at a speed several times greater than saidpredetermined speed to produce further sliding movement of said object relative to said surface in a direction normal to the direction of said first-named sliding movement, whereby the length of the path traversed by said object in moving from one end of said tube to the other is greatly in excess of the length of said tube, and contacting said object with a part fixed relative to said surface during said sliding movement, thereby to change the position of said object so that dilferent edge sections thereof are brought to bear against said surface.

3. The method of edge grinding piezoelectric crystals in batches, which comprises freely supporting the crystals upon an abrasive surface with only the oppositely disposed edge portions of the crystals bearing against said surface, slowly sliding said crystals back and forth over said surface in opposite directions, rapidly sliding said surface beneath said crystals in a different direction, and changing at random the edge portions of the crystals which bear against said surface.

4. The method of edge grinding piezoelectric crystals in batches so that rounded chamfers are produced at the edges thereof, which comprises freely supporting the crystals within a tube having an abrasive material therein so that only the oppositely disposed edge portions of each-crystal bear against the inner surface of the tube, tilt ing said tube so that said crystals are slid back and forth longitudinally of said tube, concurrently rotating said tube to slide the inner surface thereof beneath said crystals, and continuously changing at random the edge portions of each crystal which bear against the inner surface of said tube.

5. Apparatus for edge grinding relatively small objects, comprising an elongated hollow tube-like member which is adapted to have an abrasive material on the inner surface thereof and is adapted to support on said inner surface edge portions of the objects which may be disposed therewithin, means supporting said member so that one end thereof may be moved alternately above and below the other end thereof and for rotary movement about an axis which is offset from the longitudinal axis thereof, means for rapidly rotating said member about said first-named axis in order rapidly to slide said inner surface beneath the objects, and means for moving said member so that said one end thereof is alternately above and below the other end thereof at a rate which is so related to the length of said member that one of the objects will move substantially the full length of said member during each period when either end of said member is above the other end of said member.

6. Apparatus for edge grinding relatively small objects, comprising an elongated hollow tube-like member which is adapted to have an abrasive material on the inner surface thereof and is adapted end thereof is alternately above and below the other end thereof, thereby to move the objects within said member back and forth longitudinally of said member, and at least one obstruction along the inner surface of said tube adapted to be engaged by the objects in order to change the edge axis, means supporting said tubes for movement as a unit, each of said tubes being adapted to have an abrasive material on the inner surface thereof and being adapted to support on its inner surface edge portions of the objects which may be disposed therewithin, means for moving said tubes so that one end of each tube is alternately above and below the other end thereof, and means for rotating said tubes about said commonaxis.

8. Apparatus for edge grinding relatively small objects, comprising a plurality of tubes substantially equally spaced radially from a common axis. means supporting said tubes for movement as a unit, each of said tubes being adapted to have an about said common axis, thereby to slide the inner surfaces ofsaid tubes around the objects disposed therewithin, means for moving said tubes so that one end of each tube is alternately above and below the other end thereof, and means within each tube adapted to be engaged by the objects to change the edge portions of the objects which bear against the inner surfaces of-said tubes.

9. The method of edge grinding a piezo-electric crystal blank to change a functional characterisportions of the objects which bear against said I inner surface.

'1. Apparatus for edge grinding relatively small objects, comprising a plurality of tubes arranged so that the longitudinal axis thereof are substantially equally spaced radially from a common tic thereof, which comprises freely supporting the crystal along a pair of its opposed edges upon abrasive material on the insidesurfaoe of a tube. and rotating the tube at such a speed that the abrasive material on the inside surface of the tube moves relative to the freely supported crystal thereon to grind the crystal edges.

- HAL F. FRUTH. 

